In magnetic-medium-based storage devices, data can be stored on circular, concentric tracks on a magnetic disk surface. A read/write head can retrieve and record data on a magnetic layer of a rotating disk as the head flies on a cushion of air over the disk surface. When retrieving data, magnetic field variations can be converted into an analog electrical signal, which can then be amplified and converted to a digital signal for signal processing. To guarantee the quality of the information stored on and read back from the disk, the read/write head needs to be precisely positioned at substantially the center of a track during both writing and reading. A closed-loop servo system, driven by servo information embedded in a dedicated portion of the track, can be used as a reference for positioning the head.
The servo information generally defines the position of the data tracks and is generally written with great accuracy to ensure that the head servo system operates properly. The servo information can be written on each surface as a radially extending set of spokes or wedges. The portion of a servo wedge at a particular track location may contain a sync field, and index mark, a gray coded track number, and two or more fine-positioned offset bursts configured in an echelon across the track. Head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of the offset bursts.
A servo writer can be used to write the embedded servo information on the disk surface. A servo writer can include a large base (e.g., granite base) to minimize the effects of vibration. The servo writer also may use precision fixtures to hold the target drive, a precision, laser-interferometer-based actuator arm positioning mechanism to place the arms radially with respect to the axis of rotation of the disks in the drive, and an external clock head to position the servo wedges in time. Conventional servo writers are typically large in size and expensive to be manufactured. Further, as track density increases, the servo writing time required to write the servo information also increases, which can create a bottleneck in the disk drive manufacturing process.
Conventional hard disk drives (HDD) increasingly use self-servo-write (SSW) processes to write servo sectors using the same heads that are used to read/write data. In some implementations, servo patterns can be written on a machine readable medium for position control. For example, spirals can be written on a machine readable medium. A servo can use the spirals (e.g., servo on the spirals) to position heads to write servo tracks. Typically, the servo moves in a radial direction across the spirals and measures the time shifts of the spirals. For example, a time shift of a spiral can be defined as an amount of time that corresponds to a difference in circumferential distance from one radial location on a spiral to another radial location on the spiral. The servo can determine radial position measurements from the measured time shifts. The measured time shifts depend on a slope of the spiral. Therefore, the slope of the spiral can affect the placement of individual servo tracks and radial spacing between the servo tracks. Variations or inaccuracies in the slope of the spiral can result in imprecise radial spacing between servo tracks.